Magnesium treated cast irons, such as compacted graphite iron (CGI) and ductile cast iron (SG) are widely used for a variety of applications. Unfortunately, however, in comparison to conventional grey cast iron these alloys are relatively difficult to machine. This factor has prevented their application to many high volume products which require large amounts of machining. One example is automotive cylinders blocks.
Although the higher strength, stiffness and ductility of CGI and SG relative to conventional grey cast irons accounts for much the difference in machinability between these materials, other factors may also be active. One such factor is the presence of 1–5 μm diameter manganese sulfide (MnS) inclusions in grey cast iron. These inclusions are known to adhere to the cutting edge of the machining tools, thus forming a protective layer and reducing tool wear. The effect is particularly significant at high cutting speeds (400–1000 m/min) when using cubic boron nitride (CBN) or ceramic cutting materials. It is also known that high sulfur and manganese additions improve the machinability of steels. Such steel alloys are referred to in the trade as “free-machining steels”. Again, the mechanism for improved machinability relates to the formation of a protective MnS layer on the cutting insert and/or a lubricating effect of the sulfide inclusions between the cutting insert and the workpiece and/or chip.
Unfortunately, manganese sulfide inclusions are not stable in magnesium-treated cast irons due to the lower free-energy of magnesium sulfide at ironmaking temperatures (FIG. 1). Therefore, a previous patent (SE 9800750-3), incorporated by reference, disclosed a method to convert the existing MgO.SiO2 inclusions in magnesium-treated cast irons to soft calcium-bearing inclusions. However, there are no available methods for producing a magnesium-treated cast iron alloy containing soft and pliable inclusions.